The invention generally relates to radio frequency identification tags, and in particular, to a method and apparatus for selectively accessing certain radio frequency identification devices that are in close proximity to other radio frequency identification devices.
Remotely powered electronic devices and related systems are known. For example, U.S. Pat. No. 5,099,227 issued to Geiszler et al. titled xe2x80x9cProximity Detecting Apparatusxe2x80x9d, discloses a remotely powered device which uses electromagnetic coupling to derive power from a remote source and then uses both electromagnetic and electrostatic coupling to transmit stored data to a receiver often collocated with the remote source. Such remotely powered communication devices are commonly known as radio frequency identification (xe2x80x9cRFIDxe2x80x9d) devices.
It is understood that the term electromagnetic refers to systems that primarily send and receive magnetic fields and are primarily inductively coupled. It is understood that the term electrostatic refers to systems that primarily send and receive electric fields and are primarily capacitively coupled.
RFID devices and associated systems have numerous uses. For example, RFID devices are frequently used for personal identification in automated gate sentry applications protecting secured buildings or areas. These devices often take the form of access control cards. Information stored on the RFID device identifies the person seeking access to the secured building or area. Older automated gate sentry applications require the person accessing the building to insert or swipe their identification tag or card into or through a exciter for the system to read the information from the identification tag. Newer RFID device systems allow the RFID device to be read at a small distance using radio frequency data transmission technology, thereby eliminating the need to insert or swipe an identification tag into or through an exciter. Most typically, the user simply holds or places the RFID device near a base station, which is coupled to a security system securing the building or area. The base station transmits an excitation signal to the RFID device that powers circuitry contained on the RFID device. The circuitry, in response to the excitation signal, communicates stored information from the radio frequency tag to the base station, which receives and decodes the information. The information read is used by the security system to determine if access is appropriate. Also, RFID devices may be written remotely by an excitation signal appropriately modulated in a predetermined manner.
In addition to typical applications for access control of persons, RFID devices are useful in applications identifying things, such as electronic animal identification, baggage tracking, parcel tracking, inventory management applications, asset identification and tracking, and other applications. These applications involve transmitting stored information from a device to an exciter/exciter system in close proximity with the device. These applications may also involve transmitting information from the exciter to the device for storage on the device.
Earlier RFID devices and systems primarily use electromagnetic coupling to remotely power the remote device and couple the remote device with an exciter system and a receiver system. The exciter generates an electromagnetic excitation signal to power up the device and the receiver receives the signal produced by the remote device.
Earlier electromagnetic coupling mechanisms include an oscillator as part of the exciter circuitry and a coil antenna on both the exciter circuitry and the remote device that includes an electronic circuit. For example, in an earlier system, excitation circuitry is connected to a coil antenna, which radiates excitation signals that are picked up by a coil antenna mounted on the device that also contains the electronic circuit. The excitation signals energize the circuit, which then provides an information-carrying signal that is transmitted to the receiver using electromagnetic or electrostatic coupling.
One problem with the use of electromagnetic coupling between a remote device and either an exciter or a receiver has been the complexity involved in the manufacture of remote devices that employ a coil antenna. The spiral layout of a typical coil antenna makes it more difficult to produce, increases cost and also the size of the remote device. Also, the coil antennas require tight tolerances for efficient performance.
Electrostatic coupling for RFID devices has been proposed to address the problems and cost associated with electromagnetic coupling. For electrostatically coupled RFID devices, the antenna coil and/or capacitor external to the electronic circuit are eliminated and replaced with cheaper electrostatic antennas.
In considering the use of RFID devices for applications identifying items, such as in inventory control, asset management, and the like, a problem arises in the control over which devices are energized or accessed at a particular time. For example, if a number of articles, each identified by a RFID device, are stored in close proximity, such as on a group of shelves in a warehouse or supermarket, then generally energizing the areas surrounding the RFID devices will result in numerous RFID devices responding. This may not be desirable. Of course, an anti-collision protocol for electronically selecting a particular tag or group of devices somewhat alleviates the problem. However, this solution adds overhead and attendant cost. These additional costs cannot be tolerated in many areas, such as product codes or electronic bar codes. Also, anti-collision protocols alone do not scale to extremely large numbers of RFID devices, that is, the anti-collision protocol becomes more complex in proportion to the number of RFID devices simultaneously read.
Therefore, a need exists for a simple, low cost solution for activating RFID devices that are in close proximity.